Follow-up to the turn-time feature: the terminal now stamps a wall-clock
time on the turn-start / turn-end rows and an elapsed duration on
turn-end, sourced from a per-event `ts` (unix seconds) that the backend
serializes as a flattened sibling of the event tag on both the live SSE
frame and each history row.
- terminal-rendering.md: add the `.turn-time` row to the taxonomy table
+ a note on the ts source and the graceful degradation when a frame
omits ts.
- web-ui/agent.md: note the ts sibling on the emitted events, and the
time/duration suffixes on the TurnStart / TurnEnd renderings.
6b4b0f11c0